1. Field of the Invention
The present invention relates to a fluid-filled vibration damping device to be used for an automotive engine mount and the like, especially to a bidirectional attenuation type fluid-filled vibration damping device capable of obtaining a vibration damping effect both in the axial and axis-perpendicular directions.
2. Description of the Related Art
Conventionally, there has been known a vibration damping device as a kind of vibration damping coupling body or a vibration damping supporting body interposed between the members constituting a vibration transmission system to connect said members to each other in a vibration damping manner. Further, as a vibration damping device, a fluid-filled vibration damping device using the vibration damping effect based on the flow action of a non-compressible fluid sealed therein is proposed, which is applied to an automotive engine mount and the like. This fluid-filled vibration damping device has a structure where a first mounting member and a second mounting member are elastically connected to each other by a main rubber elastic body, and a pressure-receiving chamber with its walls partially composed of said main rubber elastic body and an equilibrium chamber with its walls partially composed of a flexible film are each filled with a non-compressible fluid, while said pressure-receiving chamber and said equilibrium chamber are connected to each other by a first orifice passage. Then, a relative pressure difference is produced between the pressure-receiving chamber and the equilibrium chamber at the time of vibration input in the axial direction to generate a fluid flow through the first orifice passage, thus exerting the vibration effect based on the flow action of the fluid.
Also, as a fluid-filled vibration damping device, a device called a bidirectional attenuation type is proposed wherein the vibration damping function is effective against a vibration input not only in the axial direction but also in the axis-perpendicular direction. That is, as disclosed in Japanese Unexamined Patent Publication No. JP-A-2002-227912, the device has a structure where a pair of axis-perpendicular liquid chambers are formed on opposite sides of the first mounting member in the axis-perpendicular direction with a non-compressible fluid sealed therein, said axis-perpendicular liquid chambers being connected to each other by a second orifice passage. Then, in response to a vibration input in the axis-perpendicular direction, a relative pressure difference is produced between the pair of axis-perpendicular liquid chambers to generate a fluid flow through the second orifice passage so that the vibration damping effect based on the fluid flow action is exerted.
Meanwhile, the fluid-filled vibration damping device shown in JP-A-2002-227912 is made in a configuration where a thick outer wall (54) composing a main rubber elastic body (16) undergoes shear deformation under a load input in the axial direction to induce tensile stress. Therefore, with the device mounted on the vibration transmission system, if a static support load such as the one of a power unit is input in the axial direction all the time, it is so hard to maintain the durability of the device that the static support load has to be supported by another vibration damping device.
Thus, a structure is proposed as described in U.S. Pat. No. 8,695,954 where the device undergoes a compressive deformation under an input of the static support load in the axial direction so as to reduce or avoid the effect of tensile stress. That is, in U.S. Pat. No. 8,695,954, a main rubber elastic body (18) is configured by having a first rubber elastic body (24) and a second rubber elastic body (38) which are separate elements overlapped with each other in the axial direction, and the first and second rubber elastic bodies are each configured to be compressed at the time of an input of the static support load in the axial direction, while a pair of axis-perpendicular liquid chambers (102) are formed between the overlapping faces of the first and second rubber elastic bodies.
Also, in the structure of U.S. Pat. No. 8,695,954, the second rubber elastic body arranged on the inner side in the axial direction constituting the walls of a pressure-receiving chamber (42) is made thicker than the first rubber elastic body arranged on the outer side in the same direction so that the substantially effective piston surface area that causes pressure fluctuation in the pressure-receiving chamber under the vibration input in the axial direction is determined by the size of the second rubber elastic body. However, since the second rubber elastic body arranged on the inside has a smaller diameter than the first rubber elastic body arranged on the outside, the effective piston surface area corresponding to the input in the axial direction gets smaller and internal pressure fluctuations in the pressure-receiving chamber are small, posing a risk of reducing the vibration damping performance. Meanwhile, in the structure described in U.S. Pat. No. 8,695,954, once the effective piston surface area is secured large enough in order to ensure the vibration damping performance against the input in the axial direction, the outside diameter dimension of the fluid-filled vibration damping device gets increased, which causes problems such as gained weight and a larger space required for installation thereof and so forth.